1. Field of the Invention
This invention relates generally to optical multiplexers, and more particularly to wavelength selectable optical multiplexers and de-multiplexers.
2. Description of Related Art
Optical communication systems are a substantial and fast-growing constituent of communication networks. As used herein, an optical communication system, relates to any system which uses optical signals to convey information across an optical waveguiding medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, N.Y.) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each information source, the complete signal being constructed from the signal portion collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded, e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. A WDM system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a WDM system, optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single waveguide, and de-multiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical systems. Exemplary WDM optical communication systems are described in commonly-assigned U.S. Pat. Nos. 5,504,609, 5,532,864, and 5,557,442, the disclosures of which are incorporated herein by reference.
In many applications, such as optical LANs, cable television subscriber systems, and telecommunications networks, there is a need to route one or more channels of a multiplexed optical signal to different destinations. Such routing occurs when optical channels are sent to or withdrawn from an optical transmission line e.g., for sending optical channels between a terminal and an optical bus or routing long distance telecommunications traffic to individual cities. This form of optical routing is generally referred to as optical add-drop multiplexing.
The most prevalent device used for combining and extracting wavelengths in a DWDM system is an Array Waveguide (AWG). The AWG suffers from an undesirable side effect that requires each port transmit, or receive in the case of a de-multiplexer, only one specific, pre-determined wavelength and sequential wavelengths. This is problematic in the case that one of the transmitters fails. A new transmitter of the identical wavelength must be added to that specific port. A second multiplexer design uses couplers that have the unpleasant side effect of adding -10logN to 3log.sub.2 NdB of loss at each stage of coupling.
There is a need for a DWDM device, sub-system and system with flexibility in design, configuration and degree of system refinement. There is another need for multiplexing that frees transmitters to use any port. A further need exists for flexible construction of multiplexers and de-multiplexers using different circulator port counts and interchangeable device types. There is a further need for the use of variable tunable filters working in concert to tailor a DWDM signal for gain flatness as well as other applications. Another need exists for DWDM devices, sub-systems and systems with low cross-talk.